Waterbeds have certainly grown up a lot since their peak popularity in the 70's. While they have always had some real advantages, especially in terms of pressure relief, since a liquid surrounded by a flexible bladder is equal to or better than most other materials in distributing pressure, their disadvantages, and some of the misinformation and outdated information surrounding them, have often prevented them from being a serious choice for many people.
Originally waterbeds were what is now called hardside waterbeds which was a simple vinyl bladder filled with water and put inside a hard, usually wooden frame with a supportive pedestal underneath it. Since they needed a special enclosure and support, they could not be used inside traditional bed frames. They were free flowing and the wave motion became one of the "signs of the times". They were part of a growing counterculture that easily attached itself to different and sometimes radical ideas and quickly became an icon of "cool" among a growing group of supporters.
Besides the stories that were attached to the "benefits" of the waves that had nothing to do with sleeping, the genuine benefits of sleeping on water included great pressure relief, the ability to heat the water and sleep on a regulated and even therapeutic temperature, and the ease of cleaning the sleeping surface and preventing unwanted visitors from making a home inside the mattress. The disadvantages however included the possibility of leakage, the wave motion (for those who didn't think it was so "cool"), the heating source and possible overheating or breakdown, the weight, the maintenance and water conditioning requirements, custom bedding, and the difficulty of draining and moving the mattress. In spite of this, they remained popular with a core group of believers long after their initial popularity reached a peak and declined.
While it was and still is widely recognized that waterbeds will relieve pressure very well, their ability to keep your spine aligned became the subject of hot debate with many "experts" claiming that "there was nothing like them" while others with equal "authority" insisting that they were worse than any other sleeping system. This controversy continues to this day and while each side includes countless authoritative (and not so authoritative) sources such as chiropractors, doctors, and "sleep experts", who you would think could explain the "why" behind their beliefs, most of them do little more than make the side they fall on very clear and justify it with vague statements that misuse certain terms (such as support) or publish half truths and misleading statements as being fact. Many of them have an agenda which is controlled by either the sale of waterbeds or competing mattresses. This page will attempt to correct that.
The obvious disadvantages of hardsided waterbeds led to a great deal of development in waterbed design which has improved most of its more obvious weaknesses. One of the first of these was soft sided designs which use less water, is lighter, and uses foam encasements of different types which along with a mattress ticking are made to resemble traditional mattresses and fit inside a regular bed frame using regular sheets. Some of these add "comfort layers" of foam or other materials and use the water as a core while some of them do not and are made to appeal to those who like the feel of sleeping directly on water. Through the addition of different types of internal fibers or baffle systems and separate compartments, they gained the ability to be made either waveless, semi waveless, or remain as full wave versions for those who wanted "less or more of the cool stuff". This also reduced the effect of a heavier sleeper on a lighter partner. In some with comfort layers, heating became unneccessary while heating systems were also improved for those who needed or preferred heated waterbeds. Further refinements in the flexibility of the bladder material and and the development of more flexible and "hinged" encasements made of foam combined with firmer materials, increased the overall flexibility of the sleeping surface and gave it greater contouring ability which became less of a limiting factor and improved the feel of "sleeping on water". Construction methods and seams also improved making leakage far less likely. Difficulties with draining, especially with baffled or fiber filled bladders which can shift, and the difficulty of moving them remain.
While there is no doubt that waterbeds can be exceptionally pressure relieving if they are in a flexible bladder, and there is no doubt that for many people, some of their other benefits such as temperature regulation, "sanitary" sleeping surfaces, and even the "non sleeping" benefits of the original wave motion can be attractive, the controversy about their ability to support and maintain good spinal alignment remains and most of the further development of waterbeds has centered around different ways to address this and to offset the "waterbeds have poor support" side of the controversy. Methods of addressing this have in some ways paralleled airbeds through the further development of separate sections or "zoning" that can be firmer (with more water, more fiber layers, thicker baffling, or hydraulic "water coil" systems) to in theory hold up heavier parts of the body and provide better alignment. Part of the problem with many of these was that the feel of sleeping on water was reduced.
To clarify all of this, and bring the argument back to its simplest form, mattresses have two main functions, one of which is pressure relief, and the other is spinal alignment. Since there is no doubt that waterbeds can do a superior job of the first, the rest of this page will talk about the second and hopefully get past the lack of good and undistorted information that is available. In addition to this, most of the information you will encounter confuses the idea of support itself. Because a waterbed will always form a cradle and support a certain sleeping position, some call it supportive. The problem is that while there is no doubt that it forms a cradle which supports the body well in a particular position, the question remains whether this position has good spinal alignment.
To provide some context, we will use the example of our carved wood once again (from the basic functions of a mattress page). If you are sleeping on a piece of wood that has been carved to perfectly conform to the shape of your body in a certain position, you will have pressure relief for as long as you are in that position. Your weight will be spread out over the surface of the wood and it will feel soft as long as you don't move. This is true for all materials that are denser than your body (which is one reason that air does not do as well here since you will always keep sinking into air until you reach a "hard" surface or the bladder is stretched to its limit). While this piece of wood may offer perfect pressure relief, it may not provide good spinal alignment since this would depend on whether the carved surface was contoured to your shape in the best alignment or to your shape in a more hammocked position (hips down too far) or swayback position (hyperextended lumbar). It is important to remember that perfect pressure relief can happen in a sleeping position that is poorly aligned. This is the crux of the waterbed controversy and a big part of how support has been confused with pressure relief.
So the answer to the waterbed alignment issue is really ... it depends. And what it depends on is the different weight distribution, and the unique shapes and sizes of the different parts of the person sleeping on it. In water that is not enclosed by a bladder like an ocean, buoyancy is the force that will either hold you up or let you sink in deeper into the water and this is controlled by density (weight per volume) of different parts of your body compared to water. The denser the water (like salt water compared to fresh water), the less you will sink before you are floating. This is because outside of buoyancy there is only water surface tension to stop you from sinking in so water can flow around you as you sink deeper and displace the water. In a bladder however, this changes because the water cannot flow through the bladder and around you as you displace the water. Now what determines how far each area of you sinks is the weight of everything above your points of contact per square inch (or any measure of surface area) rather than weight per cubic inch (or any measure of volume).
This force per square inch is the same thing that determines how deeply foam compresses. Unlike foam though, water does not compress at all and for every part of you that sinks in a little deeper, all the rest of you will lift up to some degree, with lighter parts lifting up more and heavier parts less. (With foam this "lifting up" does not happen since it compresses underneath you and does not affect surrounding areas). First the heavier parts will sink in and lift the lighter parts of you higher (or someone else as this includes anything on top of the mattress, even another person), and then lift slightly heavier parts a little less until every area of the body has sunk in as deeply as it can and is in balance with all the parts that have been lifted. At this point no part of you can sink any further or be raised any higher and the forces have equalized. This feels very much like floating although it is not (in floating the denser parts of you sink in and don't lift other parts even if they are lighter overall).
So the difference between a waterbed and foam or an innerspring is that with a waterbed, for every part of you that sinks in, every other part will raise a little until all the forces are in balance, with a foam, every part of you that sinks in has little to no effect on other parts. Waterbeds in larger sizes will lift the lighter parts up less than smaller waterbeds with a smaller surface area. Size makes a difference here, both in terms of the waterbed surface and the surface area of different parts of your body.
This means that when everything has equalized, if the cradle that is shaped around you in the waterbed, is similar to the surface area (depth) of a cradle that is formed around you by a softer foam or other types of comfort layers, then the pressure on each part of your body would be virtually identical between the two mattresses. The difference would be that with a waterbed, the parts of you that were lighter with a larger surface area would be higher because they were being lifted up while with a foam mattress they would be lower because of the compression of the foam. Same pressure relief, but with a different alignment of the body. Now if the waterbed spinal alignment was better than a mattress that uses other types of comfort layers (memory foam, polyfoam, latex foam, microcoils, natural fibers etc) and a different support system (latex, polyfoam, innersprings etc), then it would be the better choice between these two specific mattresses, particularly if it was better in all your sleeping positions. If your spinal alignment on another mattress is better, particularly in all your sleeping positions, then in this specific case the alternative would be a better mattress for you.
Interestingly enough, if you were to put foam that compressed inside a water bladder, you would have a mattress that was partly liquid (viscous) and partly elastic. It would be visco-elastic just like memory foam. Of course it would not be the same because memory foam doesn't have a bladder around it and because it also changes its properties in specific areas with the application of heat and pressure while the waterbed version would always be the same, but it is always interesting to connect some dots just as food for thought.
You can change how deeply parts of you sink into a waterbed and change spinal alignment by filling areas of the mattress with fiber or foam or by enclosing the water in separate smaller bladders with more or less water, some of which would hold parts of you up higher and some of which let parts of you sink in deeper. With water, you always get good pressure relief if the bladder is flexible enough but the ability to make changes to keep you in alignment at the same time and in different sleeping positions, while possible, is more difficult and restricted than with foam. With different types of foam comfort layers, pressure relief can be equal to a waterbed but there is more flexibility in getting there and keeping you in good alignment at the same time by using foams or other core materials with different ILDs, differing levels of progressive resistance, different layering, different resilience, and different elasticity.
So to answer the question ... can a waterbed provide good alignment in all your sleeping positions along with its known pressure relieving qualities?
The answer for some people is yes (particularly if your weight is more evenly distributed) and if you are one of these and you also like the pressure relief and "feel" of a waterbed then it can certainly be worth considering ... and you may be "cooler" than you think. The answer for other people is no (particularly if you have a more uneven weight distribution) and if you are one of these then it can lead to back pain and discomfort that in some cases can be severe.
In the end ... the only way to know for certain whether any particular waterbed design will be suitable for you to sleep on will be based on some careful and extended testing in the store in all your sleeping positions (with both of you on the mattress if you sleep with a partner because your partner can affect your alignment on a waterbed as well) or your own personal experience when you sleep on the waterbed.